Improve CSS for hiding the accessibility labels

This used to result in superfluous scroll bars
in WebKit/ Blink based browsers on enwiki:
https://en.wikipedia.org/wiki/Wikipedia:Village_pump_%28technical%29#Scrolling_past_the_bottom_of_the_page...

Change-Id: I4e7518262b8fdc5d3de49fff46166f7e205be9fe